Accelerated MRI Techniques: Basics of Parallel Imaging and ......Accelerated MRI Techniques: Basics...

Accelerated MRI Techniques: Basics of Parallel Imaging and Compressed Sensing

Peng Hu, Ph.D. Associate Professor

Department of Radiological Sciences PengHu@mednet.ucla.edu

310-267-6838

MRI...

● MRI has low signal levels – Polarization is PPM

● Overcome with higher fields – Improve detection

● High quality coil arrays ● Mostly body noise limited today

● MRI is slow... – Slow to encode

● Compare to digital camera! – Slow repetition times

● Relaxation time constants are long – Need contrast agents – Need faster gradients (1990s)

● Gradients are near optimal today

Gradient Encoding

● One-to-one correspondence between k-space location and MRI signal

● Speed of MRI is dependent on speed of travel in k-space

● K-space location is controlled by gradients ● One MRI signal sample at a time!

– larger volume coverage -> longer scan time

S

Waitaminute…

● Can we increase the speed we travel in k-space using higher gradients and faster switching?

● Yes, you can, but… – Peripheral nerve stimulation – Gradient amplifier power considerations – SNR considerations

NominalSlow Faster

PeripheralNerveStimulation

● Switching of gradients -> time-varying magnetic field -> electrical current -> nerve stimulation -> tingling sensation

● PNS is not dangerous, but can be disturbing ● FDA limits PNS in MRI systems -> limits in

switching speed of gradients ● Common Max slew rate: 200mT/m/ms

GradientAmplifier

● Gradient amplifiers feed large electrical currents into the gradient coil

● Gmax ∝ Current [I, amps] ● Slewrate ∝ Voltage [V, volts] ● Power=IV ● R-fold acceleration requires

● R-fold increase in Gmax ● R2-fold increase in slewrate ● Power=IV∝R3!!!

SNRLoss

● Larger sampling bandwidth -> Larger anti-aliasing filter BW -> allowing more noise power into MRI signal -> decreasing SNR!

● Common Max Sampling Rate: 500KHz (2us period)

AlternativeTechniquetoSpeedupMRI

● Reduce k-space samples

Parallel Imaging!

WhyMRIusingCoilArrays

● Increased SNR

SourcesofNoiseinMRI

● Human Body – Noise from human body is most significant at

high field ● Electronics

– Coils, Pre-Amps, amplifiers, filters, A/D ● Interference

– Less of an issue

Coil 2

Coil 4

Coil 1

Coil 3

Multi-coilReconstruction

Multi-coilReconstruction

Multi-coilReconstruction

Recommended Reading: “The NMR Phase Array”, Roemer et al, MRM 1990

IdealCoilSensitivity

Intheidealworld…

SignalEquationwithCoils

Coil Sensitivity Modulation

Coil Sensitivity

MRSignalEquation–DiscreteForm

1D Simplification

Discrete Form

2-VoxelCase

A B

4VoxelCase

A B C D

InverseProblem

Orthonormal Fourier Encoding Matrix!

4VoxelswithCoils

A B C DCoil 1 Coil 2

Over-determined!

Under-Sampling!

Sensitivity Encoding Matrix!

2X Acceleration

k-spaceUnder-sampling

kx

ky

kx

ky

FFT➠

FFT➠

SENSE

Image

Coil Sensitivity Information

Coil 2

Coil 1

Object

Unwrap fold over in image space

( )C rγ

!

SensitivityEncodingMatrix

● A huge matrix! – 256*256*32 by 256*256

● Pseudo inverse can be simplified in Cartesian sampling

● For non-Cartesian scanning, conjugate gradient methods can be used to iteratively solve the inverse problem.

● Requires prior knowledge of coil sensitivity – Errors in coil sensitivity causes artifacts

Recommended Reading: “SENSE: sensitivity encoding for fast MRI”, Pruessmann et al, MRM 1999

Coiln

* =Undersampling

Object Object*Coiln Aliased

CartesianSENSE

Coiln

* =Undersampling

Object Object*Coiln Aliased

CartesianSENSE

* =

Undersampling

Coil 1

Coil 1 Coil 2

Coil 1 Coil 2 Coil 3 Coil N

SENSERate-2

Known [n x 1]

Known [n x 2]

Unknown [2 x 1]

SENSEandSNR

● R - reduction or acceleration factor – Loss associated with scan time reduction – Typically ~1/2 N-coils

● g - geometry factor – Loss associated with coil correlation – For R=1, g=1 – For R=2, g=~1.5-2

● SNR is spatially dependent – Higher in areas of aliasing

HowFastCanWeGo?

SensitivityEncodingMatrixConditioning

● Depends on several factors – Accuracy of coil sensitivity – K-space under-sampling pattern – Coil geometry and sensitivity

● Noise is amplified during inversion – G-factor

ParallelImagingTradeoffs

1/g-MapforRate-4

∞ elements 32 elements 16 elements

12 elements 8 elements

Relative SNR Scale

G-factoranditsimpactonimage

Pruessmann et al, MRM 1999

Rate 1 2 2.4 3 4

g-m

apSE

NSE

alia

sed

1/g-factormap&Rate-4

8-channel Head coil Rate-4 (tight FOV)

OutstandingProblems

● SNR optimization – Coil design – Reconstruction algorithms

● Estimation of true coil sensitivities

CoilSensitivityEstimation

Pruessmann et al, MRM 1999

Dependenceoncoilsensitivityaccuracy

Pruessmann et al, MRM 1999

● Images reconstructed using coil sensitivity maps calculated using different order P of polynomial fitting

P=0 P=1 P=2

K-space based parallel imaging methods

Synthesizingspatialharmonics

IF

THEN

UseofHarmonics:Skippingk-spacelines

kx

ky

Coil 1 Coil 2

n1 n 2

} Δky

Whatfrequencycanwesynthesize?

● Depends on the frequency component of coil sensitivities

● Extreme Example:

yCoi

l sen

sitiv

ity

C1(y) C2(y) Ccomp(y)

SpatialHarmonics

Sodickson et al, MRM 38:591-603

SMASH

ImageObjectCoil 2

Auto- Calibration

Coil 1

Reconstruct Missing k-space

Auto-CalibrationCoil 1 Coil 2

VariationsofSMASH

Comparisonb/wSENSEandSMASH

● SMASH is a special case of SENSE – Spatial harmonics allow for reduction of

encoding matrix ● SMASH does not require direct measurement of

coil sensitivity – Auto-calibrating

● SENSE fails when FOV < Object size

ParallelImagingSummary

● Parallel imaging uses coil sensitivities to speed up MRI acquisition

● Cases for parallel imaging – Higher patient throughput, real-time imaging,

imaging for interventions, motion suppression ● Cases against parallel imaging

– SNR starving applications, imaging coil map problems

CompressedSensingMRI

● CS is a method complimentary to parallel imaging to speed up image acquisitions

● Two requirements – Sparsity in a transform domain – Random under-sampling

● To the board…

IntroductiontoCS

Lustig MRM 2007

IntroductiontoCS

Lustig MRM 2007

TypesofSparsity

● In image domain – CE-MR Angiography

● In temporal domain – cine cardiac MRI

● In both temporal and image domain – Dynamic CE-MRA – DCE perfusion

● …

SparsityinMRAimages

DCEMRAandPerfusion

● Background Subtraction before CS – Enhanced sparsity, higher temp. resol.

Storey, Lee, NYU, ISMRM 2010

Systole Diastole- = Diff.

Sparsity in Time

Questions?● Peng Hu, Ph.D. ● 300 Medical Plaza B119 ● 310 267 6838 ● penghu@mednet.ucla.edu ● http://mrrl.ucla.edu/meet-our-team/hu-lab/